lavc: Correct the description of pkt_dts

The description as updated in 6e5c44b0f made it sound like it is
handled exactly like the pts, which it is not. Clarify that it
is the packet that triggered returning the frame instead of just
the last avpacket (in case of frame threading, it won't be the
dts of the last packet).
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/avcodec.h

@@ -1095,7 +1095,7 @@ typedef struct AVFrame {
     int64_t pkt_pts;
 
     /**
-     * dts copied from the AVPacket that was decoded to produce this frame
+     * dts copied from the AVPacket that triggered returning this frame
      * - encoding: unused
      * - decoding: Read by user.
      */